The present invention relates to a pulse width modulation d.c. servo motor driving circuit of a d.c. servo motor driving device which is used for positioning a movable member in an industrial robot or a machining apparatus or the like.
FIG. 1 shows a circuit diagram of a conventional pulse width modulation driving circuit for a d.c. servo motor and the operation of the circuit will be described in conjunction with the circuit diagram of FIG. 1, and FIGS. 2 and 3 which show waveforms in the circuit shown in FIG. 1 and the fly-wheel current.
In FIG. 1, reference numeral 1 designates a trangular wave oscillator and reference A denotes a driving command signal. Resistors 2 to 7 are used for the pulse width modulation. The values of these resistors 2 to 7 are determined in such a way that when the level of the driving command signal A is zero, the pulse width of a forward rotation driving signal B is coincident with that of a reverse rotation driving signal C at small pulse widths. In this state, a forward rotation driving current for a d.c. motor 18 provided by transistor switches 11 and 12 driven by the forward rotation driving signal B and a reverse rotation driving current provided by transistor switches 10 and 13 driven by the reverse rotation driving signal C are balanced in level to stop the operation of the d.c. servo motor 18. These values are determined in accordance with the following formula: EQU V.sub.p /R.sub.2 =V.sub.M /R.sub.5 &lt;V.sub.TM /R.sub.4 =V.sub.TP /R.sub.7
wherein:
R.sub.2 to R.sub.7 are the resistance values of the resistors 2 to 7, respectively,
V.sub.TP is the positive maximum value of the triangular wave oscillator 1,
V.sub.TM is the negative maximum value thereof,
V.sub.p is an output voltage of a positive voltage source and
V.sub.M is an output voltage of a negative voltage source.
In general, since V.sub.p =V.sub.m and V.sub.TM =V.sub.TP EQU V.sub.p lR.sub.2 &lt;V.sub.TM /R.sub.4 EQU R.sub.2 =R.sub.5 EQU R.sub.4 =R.sub.7
Assuming that t.sub.max is given at the time of the maximum driving command voltage V.sub.A : ##EQU1## wherein:
T is one period of the trangular wave oscillator 1,
V.sub.p =V.sub.M and V.sub.TM =V.sub.TP.
The pulse width modulation of the forward rotation driving pulse signal B and the reverse rotation driving pulse signal C is carried out as described above, the transistor switches 10 to 13 being rendered operative, and the current corresponding to the driving command signal A being supplied to the d.c. servo motor 18.
However, in the conventional system described above, due to the inductance component of the armature of the motor and the inductance inserted in series with the motor, the system does not effectively utilize the fly-wheel current flowing through the motor. As a result, the magnitude of the ripple in the motor current is large, and the switching loss is also large. These disadvantages will be described with reference to FIG. 3 in which the equivalent circuit of the motor and the fly-wheel current are illustrated.
The driving current from a driving power source 19 flowing through the transistor switch 10, the motor 18 and the transistor switch 13 is shown by a two-dot chain line and this current is the driving current flowing through a closed loop including the transistor switches 10 and 13. When the transistor switches 10 and 13 ar turned OFF, the fly-wheel current flowing through the motor 18, a diode 15, the power source 19 and a diode 16 as shown by a one-dot chain line starts to flow. The motor 18 is shown by the corresponding equivalent circuit represented by a resistor 20, an inductance 21 and a voltage source 22. The resistor 20 represents the resistance of the armature, the inductance 21 represents the armature inductance and the voltage source 22 represents the voltage produced by the rotation of the rotor. Therefore, there are two voltage barriers in the fly-wheel current path due to the back EMF and the voltage source. The product of the current flowing by overcoming the back EMF barrier and the back EMF is changed into a mechanical energy of the motor. The product of the current flowing by overcoming the voltage source barrier and the voltage source is changed into a voltage source energy or becomes an energy loss in the voltage source. In this case, the voltage source has a higher level than the back EMF. Due to the voltage barrier in the fly-wheel current path, the fly-wheel current is rapidly decreased so that the response speed for the driving command signal is high. However, the form factor (ripple) of the motor current is lowered and thus the efficiency of the motor is also lowered. Furthermore, since the energy stored in the armature inductance of the motor is also changed into the voltage source energy or is lost, the total energy efficiency including switching loss is lowered.